Voltage comparator indicating two input signals equal employing constant current source and bistable trigger



Sept. 18, 1962 T P. BOTHWELL 3 VOLTAGE COMPARATOR INDICATING TWO INPUTSIGNALS EQIjAL EMPLOYING CONSTANT CURRENT SOURCE AND BISTABLE TRIGGERFiled May 27, 1959 /5 COMPARATOR FLIP FLOP SWEEP I "I 50 GENERATORREFERENCE VOLTAGE SOURCE CONSTANT 1 cuRRENT-\ SOURCE 6 F l G. I

Theocggre Paul Borhmuell AT ORNEY United States Patent Ofifice 3,,fi54,91 ii Patented Sept. 1 8, 1 962 3,054,910 VOLTAGE COMPARATOR INDICATINGTWO IN- PUT SIGNALS EQUAL EMPLOYHNG CGNSTANT CURRENT SGURCE AND BISTABLETRIGGER Theodore Paul Bothwell, Watertown, Mass, assiguor to EPSCG,Incorporated, Boston, Mass., a corporation of Massachusetts Filed May27, 1959, Ser. No. 816,330 4 Claims. (U. 307-885) This invention relatesgenerally to an electronic device for comparing the amplitudes ofelectrical signals and more particularly pertains to a highly sensitivevoltage comparison circuit for determining the time at which twoelectrical signals attain a predetermined voltage.

Electronic devices are known for comparing two electrical signals andproviding an output signal indicative of the time when the comparedsignals are equal in voltage. Those devices are generically termedvoltage comparators. A voltage comparator is a type of non-linearcircuit employed to ascertain the exact time at which an input signal,which may be an arbitrary waveform, attains a reference voltage level.The distinction between voltage comparator circuits andvoltage-selection or clipping circuits is that in a comparator circuitthe reproduction of any part of the input signal waveform is not anobjective. Frequently, the output of a comparator circuit is a largeamplitude, short-duration pulse, whatever the input signal waveform maybe, which occurs at the instant the input signal voltage amplitudereaches the reference voltage and is otherwise independent of the inputsignal. Voltage comparators are extensively used for timing purposes inradar systems and are also extensively used for converting analoginformation to digital form in data processing machines.

Conventional voltage comparator devices are limited in utility becauseof a lack of sensitivity and a tendency toward instability manifested byjitter. Jitter is caused by the failure of those electronic devices torespond consistently to the occurrence of the voltage equality conditionof the compared signals. For example, such a device may respond when thetwo signals are within 40 millivolts of equality and at a later timerespond when the two signals are within 25 millivolts of equality. Thatis, the device may respond at any time when the compared signals arewithin 40 to 25 millivolts of equality with the result that where thecondition of signal equality is periodically repeated, the output of thedevice is not periodic but rather is aperiodic and, in eifect, jittersabout the periodic value.

Electronic voltage comparator devices which provide an output indicativeof the time when two electrical signals are equal in voltage areemployed in digital voltmeters. The term digital voltmeter designates aninstrument for measuring the voltage of an electrical signal anddisplaying the numerical value of the measured signal, not by means ofthe customary pointer and calibrated scale, but rather, by directlypresenting the numerical digits either visually in a window of theinstrument, or electrically at designated output terminals. Inprinciple, the digital voltmeter operates by counting the number ofcycles of oscillation of a stable oscillator which occurs between thetime that a repetitively generated signal attains a reference voltagelevel and the time the generated signal becomes equal to the voltage ofthe measured signal. Jitter introduced by the electronic device fordetermining the time of voltage equality of the generated and measuredsignals is manifestly undesirable since it causes the last decimal digitin the numerical value presented by the digital voltmeter to fluctuaterapidly.

It is an object of the invention to provide an improved electronicdevice for comparing two electrical signals and 2 generating an outputsignal indicative of the time of voltage equality of the comparedsignals.

Another object of the invention is to provide an electronic voltagecomparison device that is highly sensitive to a small difference involtage between compared signals and is capable of amplifying thedifferential voltage to obtain an improvement of comparison accuracy.

A further object of the invention is to provide an electronic voltagecomparison device which has a high degree of stability, that is, freedomfrom jitter and the effect of voltage drift, coupled with sensitivity tovery small differences in voltage between compared signals.

A subsidiary object of the invention is to provide electronic voltagecomparison apparatus exclusively employing solid state amplifying andswitching devices arranged in a manner causing the leakage currentsthrough those devices to offset one another.

The invention resides in a comparator amplifier which determines with ahigh degree of accuracy the time when two electrical signals becomeequal in voltage and generates an output signal having a definition oftime of equality to better than one microsecond. The comparatoramplifier employs transistors arranged so that the effects of leakagecurrents in certain of the transistors are offset by the leakagecurrents in other of the transistors. Inherent advantages of theinvention are that matching of the characteristics of the transistors isunnecessary and the effect of voltage drift is minimized by thedilferential amplification obtaining throughout the device.

More specifically, the invention contemplates arranging a pair oftransistors and a constant current generator to cause the constantcurrent to flow through one of the transistors when the two electricalsignals are unequal and to cause half the constant current to bediverted into the other transistor when the two signals aresubstantially equal in voltage. Where, for example, the two inputsignals are within one millivolt of equality, one-half of the constantcurrent is diverted to the other transistor. The diversion of currentdue to this small difference in voltages results in an amplification ofthe differential voltage, that is, a drop in voltage ensues across theload impedance of one transistor and a corresponding rise in voltageoccurs at the load impedance of the other transistor so that the smalldifference in input signal voltages is magnified as a difference involtage drops across the load impedances of the transistors. A triggercircuit formed by a second pair of transistors is provided, the triggercircuit having an initial state in which one of the transistors isconducting in saturation and the other transistor is cut off. Thetrigger circuit is directly coupled to the first pair of transistors andis caused by the amplified differential voltage to change to a secondstate in which the condition of the second pair of transistors isreversed. Positive feedback or regeneration is incorporated in thetrigger circuit to cause that circuit to switch more rapidly from itsinitial state to its second state.

The arrangement, construction, and operation of the invention can bemore fully understood by reference to the following detailed descriptionwhen considered in conjunction with the appended drawings wherein:

FIG. 1 illustrates in schematic form a preferred embodiment of theinvention;

FIG. 2A represents the output signal of the sweep generator plottedalong a time axis;

FIG. 2B is a waveform derived at junction 25 of FIG. 1; and

FIG. 2C is a waveform derived at junction 42 of FIG. 1.

Referring to FIG. 1, there is shown in schematic form a preferredembodiment of the invention employing solid state devices, in this caseP-N-P transistors, for carrying forward the purpose of the invention.The first stage of the device includes two transistors 1, 2, havingtheir respective emitters 3, 4 connected together and to a constantcurrent genuator 5. The constant current generator may be any sourcewhose internal impedance is high compared to the impedance of its load.In this embodiment, the apparatus within the block 5, by way of example,is a transistor 6 having its emitter 7 connected through a resistor 8 toa source of positive potential impressed at terminal 9. A bias voltageis impressed at terminal 10 on the base 11 of transistor 6, the biasvoltage being of such polarity as to forwardly bias transistor 6 so thata current IBIAS flows to the collector 12. The collector 13 oftransistor 1 is connected through load resistor 14 to a terminal 15 atwhich a negative potential is impressed to bias the collector in thereverse direction. In like manner, the collector 16 of transistor 2 isconnected through load resistor 17 to terminal 15 whereby =to reverselybias that collector. The base 18 of transistor 1 is connected to theoutput of a sweep generator 19. Base 20 of transistor 2 is connected toa reference voltage source 21 which may be ground, or a stable voltagelevel above or below ground. The output signal of the sweep generator isthe ramp voltage, shown in FIG. 2A. The upper level 22 of the signal issufiiciently positive with respect to the reference voltage impressed onbase 20 to cause transistor 1 to be cut-off so that the current IBIASsupplied by the constant current generator is compelled to flow throughtransistor 2. The initial state of this circuit is then, that the signalfrom sweep generator 19 holds transistor 1 cut-off and the entirecurrent IBIAS from the constant current generator flows throughtransistor 2. As time t shown in FIG. 2A, the voltage of the generatorsweep signal commences to decrease at a constant rate along the ramp 23and at time t the voltage at point 24 0.1 the ramp becomes equal to thereference voltage, the reference voltage being assumed to be at the Zeroor ground potential for ease of exposition. At this time the rampvoltage equals or is very nearly equal (within one millivolt) to thereference voltage and both transistors are forwardly biased to the sameextent, so that the current I from the constant current generator 5divides equally, half of the current flowing through transistor 1 andthe other half flowing through transistor 2. As the ramp voltagecontinues to drop, the entire current flows through transistor 1 andtransistor 2 is cut-off. The first stage remains in this state untiltime t when the ramp voltage is being returned to its maximum positivevoltage.

With regard to the magnitude of the IBIAS current,

this current is chosen to be as small as possible consistent with therequirement that it be large compared with collector diode leakagecurrent. That is, the IBIAS current should be in the order of forty orfifty times larger than the collector diode leakage current. Thus, ifthe collector diode leakage current is 5 microamperes, IBIAS should bein the order of 200 microamperes. The IBIAS current is chosen to besmall because it is important to minimize the heat required to bedissipated by transistors 1 and 2. This follows from the fact that thebase-emitter junction bias voltage is dependent upon temperature to theextent that a change in temperature of one degree centigrade causes achange in bias voltage of approximately two millivolts. Any change, dueto temperature, in bias voltage at the base-emitter junction of one ofthe transistors 1, 2, which does not simultaneously eifect an equalchange in bias voltage at the other of those transistors, causes animpairment in the accuracy of circuit performance. It is, therefore, adesideratum that the heat dissipation requirements imposed upontransistors 1 and 2 be held to a minimum. However, the effective voltagegain of the first stage is approximately proportional to the magnitudeof the IBIAS current, and it is desirable to have appreciable gain inthe first stage in order to amplify the initially small differentialvoltage. Therefore, the IBIAS current must be a compromise between thenews A. sity for using a small current to minimize the heat dissipationof the transistors and the desire to increase the voltage gain byincreasing the IBIAS current.

Transistors 26 and 27 and their associated elements form a trigger stagefrom which the output of the device is taken at junction 42 throughcoupling capacitor 28 at terminal 29. The emitters 30 and 31 of thosetransistors are connected through a common resistor 32 to a source ofpositive potential impressed at terminal 33. The collectors 34 and 35 oftransistors 26 and 27 are connected through resistors 36 and 37 to theterminal 15 at which the source of negative potential is impressed, theterminal 15 being convenient for that purpose, which reversely biasesthe collectors of those transistors. Base 38 of transistor 26 isdirectly coupled to collector 13 of normally off transistor 1 and base39 of transistor 27 is directly coupled to collector 16 of the normallyon transistor 2. In the trigger stage, therefore, transistor 26 isnormally forwardly biased to conduction while transistor 27 is cut-off.That is, at time t FIG. 2, transistor 26 is conducting in saturationwhile transistor 27 is cut-off. The initial state of the trigger stageis deter mined by conditions existing in the first stage at time t Thiscan be more readily appreciated by considering that because transistor 1is cut-off at time t the voltage at its collector 13 is the voltageimpressed at terminal 15 reduced by the drop due to the base current oftransistor 26 and the collector leakage current which flow throughresistor 14, that voltage being directly coupled to base 38. The forwardbias on transistor 26 is then the difference between the negativepotential at collector 13 and the potential at the emitter 3i Transistor2, on the other hand, is conducting the IBIAS current, so that thevoltage at its collector 16 and junction 25 is more positive to theextent of the voltage drop caused by the collector current flowingthrough resistor 17. Base 39 is directly coupled to collector 16 at thejunction 25 and the potential at that collector is sufiiciently positivewith respect to emitter 31 to bias base 39 reversely so that transistor31 is cut-off. Hence, the initial state of the trigger circuit is suchthat transistor 27 is cut-off and transistor 26 is conducting asaturation current. At time t when the ramp voltage is equal to thereference voltage, the sudden diversion of half the IBIAS current totransistor 1 causes a rise in voltage at its collector 13 while acorresponding drop in voltage ensues at collector 16 due to the decreasein current, as indicated by the waveform of FIG. 2B. The base oftransistor 27, because it is directly coupled to emitter 16, tends tobecome biased in the forward direction whereas the base of transistor 26tends to become reversely biased. As soon as transistor 27 commences toconduct, the voltage at its collector rises, this rise in voltage beingcoupled through capacitor 40 to base 38, so that a regenerative efiectoccurs which quickly causes transistor 26 to be cut off and transistor27 to be biased to conduct a saturation current. This switching actionof the trigger circuit is extremely rapid and a reversal of its initialstate occurs in a short time. The output, derived from the collector oftransistor 27, is a negative going wave having a steep wave front 41 asindicated in FIG. 2C. The negative going wave, at junction 42, iscoupled through the DC. blocking capacitor 28 to output terminal 29. Thenegative going wave may be differentiated by conventional means and thedifferentiated pulse obtained may be used as a trigger to actuate othercircuits, the particular use to be made of the trigger pulse beingbeyond the scope of this invention.

At time t when the ramp voltage is being returned to the positivevoltage level 22, the first stage and the trigger stage are caused toreassume their initial conditions, that is, the condition Where theIBIAS current flows through transistor 2, saturation current flows intransistor 30, and transistors 1 and 31 are cut-off. The device is thenready for the next cycle of operation which commences when the sweepgenerator again furnishes a ramp voltage. Where the invention isemployed in a digital voltmeter, it

is contemplated that a ramp voltage will be repetitively generated atperiodic intervals.

The direct coupling of the base of transistor 26 to the collector oftransistor 1 and the similar direct coupling between transistors 27 and2 is an important consideration in the invention because thisarrangement results in improved circuit stability over a relativelylarge temperature range and permits the use of unselected transistors.That is, it is not necessary that the transistors be selected for theirleakage current (I characteristics but the directly coupled transistorsshould not have radically different characteristics or employ greatlydiiferent resistance values. The strong leakage current cancellationoccurring in directly coupled transistors which form similar stages canbe appreciated by considering that the leakage current (I flowing fromthe base to the collector of transistor 1 flows directly into the baseof transistor 26 and to the collector of that transistor so that no 1,,current enters any of the circuit resistances in the stage formed bytransistor 1. That is, one may imagine a fictitious current generatorconnected between base 18 and collector 13 of transistor 1 and a similarcurrent generator connected between the base and collector of transistor26, those two current generators supplying the L currents required. Thecurrent out of 1 generator of transistor 1 flows into the I generator oftransistor 26 so that no I current is drawn through any resistorassociated with the transistor 1 if the L, currents generated by bothare equal. Hence the stability of the directly coupled transistors isimproved by the cancellation of the I currents.

While the preferred embodiment of the invention has been described, itis to be understood that modifications which do not depart from theessence of the invention can be made and, indeed are apparent to thoseknowledgeable in electronics. For example, N-P-N, surface barrier, orother types of transistors may be substituted for the P-N-P transistorsillustrated and the bias voltages arranged to accord with the use ofthose other types. It is also apparent that an output signal may bederived from the collector 34 of transistor 26. In the trigger stage, asa further example, feedback from one transistor to the other may beaccomplished by other arrangements. Therefore, it is intended that thescope of the invention not be limited to the precise embodimentdisclosed herein, but rather, that the invention be construed inaccordance with the appended claims.

What is claimed is:

1. An electronic device for producing an output signal indicative of apredetermined relationship between the amplitude of a first input signaland a reference voltage comprising, a constant current generator, a pairof current amplifying elements, means for applying said first inputsignal and said reference voltage separately to different ones of saidelements to cause the constant current of said generator to flow intoone of said elements when said input signal bears other than saidpredetermined relationship to said reference voltage and for diverting apart of said current to the other of said elements upon the occurrenceof said predetermined relationship, and a trigger circuit responsive toa change in current at the outputs of said elements whereby thediversion of said constant current causes actuation of said triggercircuit.

2. An electronic comparator comprising, a generator for supplying aconstant current, a pair of semiconductor current amplifying elementsconnected to the constant current output of said generator, meansincluding a source of reference voltage for biasing said elements tocause said constant current to How into one of said elements, said meansfurther including a signal source for diverting a part of said constantcurrent to the other of said elements when the signal from said sourcebears a predetermined voltage relationship to said reference voltage, apair of load impedances, each of said elements having its outputconnected to a diiferent one of said load impedances, and a triggercircuit responsive to a change in current at the outputs of saidelements whereby the diversion of said constant current from one of saidelements to the other causes actuation of said trigger circuit.

3. A voltage comparator comprising, a constant current generator, a pairof semiconductor amplifying elements connected to the constant currentoutput of said generator, a signal source connected to one of saidamplifying elements, a reference voltage source connected to the otherof said amplifying elements, said signal source and said referencesource biasing said elements to cause the constant current output ofsaid generator to flow into one of said elements, said signal sourcebeing adapted to furnish a varying voltage signal whereby a part of saidconstant current output is diverted to the other of said elements whenthe amplitude of said varying signal is equal to the voltage of saidreference source, a trigger circuit having a pair of semiconductoramplifying devices, and each of said devices being directly coupled to adifferent one of said elements to provide cancellation of leakagecurrents.

4. A voltage comparator comprising, a source of constant current, firstand second transistors having their emitters coupled to said currentsource, a signal source coupled to the base of said first transistor, asource of reference potential coupled to the base of said secondtransistor, impedance means connecting the collectors of said first andsecond transistors to a source of biasing voltage, third and fourthtransistors forming a trigger stage, the base of said third transistorbeing directly coupled to the collector of said first transistor, thebase of said fourth transistor being directly coupled to the collectorof said second transistor, a regenerative coupling means between saidthird and fourth transistors, and means connecting the emitters andcollectors of said third and fourth transistors to said source ofbiasing voltage.

References Cited in the file of this patent UNITED STATES PATENTS2,772,359 Modiano Nov. 27, 1956 2,831,986 Sumner Apr. 22, 1958 2,846,594Pankratz Aug. 5, 1958 2,903,605 Barney et al Sept. 8, 1959 2,909,676Thomas Oct. 20, 1959 2,909,678 Jensen Oct. 20, 1959 2,920,215 Lo Jan. 5,1960 2,933,623 Jones Apr. 19, 1960 FOREIGN PATENTS 753,689 Great BritainJuly 25, 1956 OTHER REFERENCES Transistor Circuit Engineering, by Shea,1957, published by Wiley & Sons, New York, page 152.

